Group III nitride crystals are variously used as, for example, substrates for light-emitting devices such as light-emitting diodes (LED) and semiconductor lasers (laser diodes, or LD). Of these, GaN crystals are useful as substrates for blue light-emitting devices such as blue light-emitting diodes and blue semiconductor lasers, and are currently under active investigation.
When a Group III nitride crystal is to be used as a substrate, the crystal-grown Group III nitride crystal must be processed to the substrate shape. For example, when disk-shaped substrates are to be fabricated, the grown Group III nitride crystal is peripheral ground with a crystal grinding tool to give the crystal a round cross-sectional shape. In addition, slicing is frequently carried out to render the crystal to a desired size.
It is known that, in Group III nitride crystals, residual stress arises at the interior of the crystal as crystal growth proceeds, and that such stress induces warping of the crystal. In particular, when a Group III nitride crystal that has been grown on a different type of substrate is separated from the different type of substrate, warping sometimes becomes very apparent. It has been proposed that heat treatment be carried out in order to reduce such warping (see Patent Document 1). This patent publication states that warping is reduced by making the difference in dislocation density between the substrate side of the Group III nitride crystal and the opposite side smaller through heat treatment. Specifically, mention is made of heat-treating a GaN layer at 1200° C. for 24 hours, at 1400° C. for 10 minutes, and at 1600° C. for 2 hours.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-277195